Muscle Oxygen Monitor Training With Coach Aaron Davis

Based on Episode 53 of the Just Fly Performance Podcast, a conversation between host Joel Smith and Aaron Davis of Train Adapt Evolve.

A muscle oxygen monitor gave Aaron Davis a window into something coaches usually have to guess at: what is actually happening inside a working muscle. Davis, a sports performance and health coach with Train Adapt Evolve in Austin, spent years as a track athlete and coach before he started reading blood oxygen data during tempo runs, sprints, and lifts. What he found reshaped how he decides who can handle a running volume, which day is right for a heavy strength session, and when a set is doing the metabolic work it is supposed to. The through line of this episode is simple: the same athlete on paper can be two different athletes internally, and a monitor helps you tell them apart before you overcook one of them.

Key Takeaways

• A muscle oxygen monitor reads oxygen inside the muscle, not just heart rate. Davis watches the percent of oxygen in the muscle drop the instant work starts, then tracks blood flow trends to spot when a vessel is occluding.
• Occlusion tells you who can tolerate tempo. If an athlete chokes off blood flow at race pace, high volumes of extensive tempo become a stress they cannot recover from, and the monitor shows it before HRV falls apart.
• The monitor doubles as a recruitment check in the weight room. When a lift that normally occludes suddenly does not, that is a signal to move the heavy strength work to another day.
• Metabolic stress is measurable. Muscles grow from tension, damage, and metabolic stress; desaturating oxygen at a target rep count confirms you actually reached the metabolic trigger.
• Length tension drives exercise selection. Sticking points and joint angles decide when Davis reaches for isometrics, heavy eccentrics, or a quarter squat instead of a full range lift.

What a muscle oxygen monitor actually measures

The device Davis uses is a Moxy monitor, which reads the percentage of oxygen in the blood inside the muscle itself. That is a different question from what a heart rate strap answers. The first thing it reveals is that oxygen is not saved for later.

When athletes do any type of work, oxygen will decrease immediately. And that might fly in the face of some anaerobic aerobic stuff, but you use O2 immediately.

From there, Davis watches blood flow trends to find occlusion, the point where a muscle contracts hard enough to squeeze its own blood supply. He uses a plumbing picture to explain it.

If somebody occludes, just like if you hold the very end of a hose, you’re gonna have buildup on the other side of the hose. So when we see blood flow trends go up, we know that there’s usually a venous occlusion, or you can have an arterial occlusion where blood flow just kind of ceases and it plateaus.

For a coach, the practical value is that this is objective. Instead of guessing whether an athlete is working hard or working smart, you can see the internal cost of a rep scheme or a running distance and adjust from there.

Reading occlusion trends to individualize tempo and speed endurance

Occlusion is where the monitor earns its keep for running-based training. Davis frames the whole tempo debate around it: if an athlete cuts off fresh blood while still working, the rest of that effort turns into a large anaerobic tax.

If you occlude, that means while you’re doing work, you’re not getting the free fresh blood into the muscle. You’re only using whatever you came in with, and so chances are you’re gonna deplete that O2 from the muscle really fast.

He learned this the hard way by prescribing extensive tempo the same way for everyone. On athletes who occluded, the running volume drove bad heart rate variability trends and a stress reaction that showed up later in the training year, even when the workout itself looked fine on the day. His rule of thumb reframes the goal of a hard session.

How can we get in and get exactly what we need and then get out before it’s almost like robbing a house, right? Before we get caught.

The application for a distance or long-sprint coach is to stop treating tempo as a fixed dose. Two athletes running the same 250 meters can be under very different internal loads, so the monitor lets you keep the volume that helps one athlete while pulling it back for the one it quietly breaks down.

Using the monitor to time max strength days

Davis carries the same logic into the weight room, where occlusion becomes a proxy for how well an athlete is recruiting muscle on a given day. Because most of his athletes hit the track before they lift, they arrive at a max strength session with neural fatigue already banked. The monitor tells him whether the tank is there.

If their trend in the weight room, say, if they’re above 85% is an occlusion, and then all of a sudden that day they’re on the weight and they’re like, “Man, it feels heavy,” and then I don’t see an occlusion, it’s almost like you have a peripheral monitor on that muscle to see, oh, maybe our recruitment’s not as good.

When the expected occlusion does not appear, Davis treats it as permission to change the plan rather than grind through a session that will not deliver the intended adaptation.

Now do we do that workout or do we switch it up? Maybe you switch it up and you wait till the next time where he can actually create that tension that you need.

For coaches without a monitor, the transferable idea is autoregulation with evidence: heavy strength work only pays when the athlete can actually generate the tension, and forcing it on a flat day buys fatigue without the training effect.

Muscle length tension relationships and sticking points

Before the physiology monitoring, Davis built his eye on muscle length tension relationships, the idea that a muscle produces different force at different lengths and joint angles. He points to the squat as the clearest example: sticking points near the bottom or just past ninety degrees expose where an athlete is weak. He also reads it in how an athlete organizes a jump, and he told a story about a 400 meter hurdler who dropped into very deep knee flexion to jump but raced upright, a mismatch that tracked with recurring hamstring and low back trouble. His fixes target the specific range.

Some intensive isometrics are usually kind of my go-to, or eccentrics in certain ranges. You can even load up a rack pull and just have them pull at those ranges for six to ten seconds. Eccentrics, where you have a heavy eccentric load only down to maybe a quarter squat depth.

He is careful not to let range specificity become a cult. Full range mobility work belongs early in the year, and the shorter, stiffer ranges come forward as competition nears.

You always want the ability to squat deep. Full range motion type stuff or exercises can definitely be done early on, and maybe when it gets closer to the competition, now you’re looking at things more of that quarter squat.

His warning for weightlifting-influenced programs is that bottom-position bouncing is a hidden cost. Teaching an athlete to stop and hold at ninety degrees, then still lift heavy, protects the joint over a career.

It’s almost like they’re bouncing off their ligaments in the bottom all the time. One of my big keys with helping them out a lot of times is making them stop right at 90 and still try to lift heavy, and kind of look at that as like a 1RM.

Metabolic stress, hypertrophy, and selective fast twitch growth

Davis ties his muscle oxygen data directly to how muscles grow. He works from the familiar three mechanisms and uses the monitor to confirm two of them in real time.

If we think of the three ways muscles grow, it’s muscle tension, muscle damage, and metabolic stress. It’s kind of nice having the Moxy because you can really see the tension, and metabolic stress would be if we can desaturate O2.

That turns a vague hypertrophy prescription into a measurable target. If an athlete only desaturates at ten reps with a given load, stopping at eight leaves the metabolic trigger on the table. The same occlusion reading hints at fiber makeup, because a more fast twitch athlete tends to occlude sooner as a percentage of their one rep max. Davis calls selective hypertrophy of the fast twitch fibers a rare prize, and cites the classic example of shot putter Werner Gunther.

Through his training, he was able to hypertrophy the fast twitch fibers. Selective hypertrophy, which is probably the Holy Grail of coaching right there.

He also refines what coaches should mean by tension. The aim is not a rigid brace but a quality that switches on and off fast.

We want this very kind of fluid tension that relaxes just as fast as it contracts. That’s kind of more the Holy Grail, in the sense of fluidity, hard, high tensions, and then right back out to really fast relaxation.

Training cycle planning: blend the qualities, respect the pencil

Ask Davis how max strength, speed, and hypertrophy fit into a training cycle and he rejects the long single-quality block. He wants the qualities running together, with the emphasis shifting rather than the other qualities disappearing.

I think they all kind of synergistically work together. I think the best coaches usually have them all playing at the same time, and maybe just over emphasizing one or the other.

The risk he watches for is letting a hypertrophy block run so long that it degrades the coordination and neural drive a sprinter needs on the track. His filter is the biggest return for the smallest cost, worked out session by session instead of locked into a calendar written months ahead. That leads to his bluntest piece of advice about the written plan.

The big thing for coaches is just realize that whatever’s on the paper, you don’t have to do. If you have four repeats down, maybe just two is gonna be good that day. Too many times we’re beholden to the pencil, and we just need to scrap it half the time.

Speed endurance fits the same test. Davis wants to know how much of a rep an athlete spends in real anaerobic compensation, because that is what dictates the true cost.

Do we know how long speed endurance needs to be for each athlete? So if I have a 400 meter athlete that maybe depletes O2 at the 150 meter mark and then is now going in a 200, now he has to go 100 meters and he’s bottomed out.

Frequently asked questions

What is a muscle oxygen monitor?
It is a wearable sensor, in this episode a Moxy monitor, that reads the percentage of oxygen in the blood inside a working muscle. Davis uses it to watch oxygen drop the moment work begins and to track blood flow trends that reveal occlusion, giving a coach a direct look at the internal cost of a set or a running interval.

What does muscle occlusion mean in training?
Occlusion is when a muscle contracts hard enough to restrict its own blood supply. Davis distinguishes venous occlusion, where blood pools on the far side, from arterial occlusion, where flow plateaus and nearly stops. Whether and when an athlete occludes shapes how much tempo or heavy work they can absorb.

How do you individualize tempo and speed endurance work?
Davis reads occlusion and desaturation to see how each athlete handles running volume. An athlete who occludes at race pace pays a heavy anaerobic tax, so he trims their extensive tempo, while an athlete who stays open can tolerate more. The goal is to get in, get the adaptation, and get out before the stress compounds.

What are the three ways muscles grow?
Muscle tension, muscle damage, and metabolic stress. Davis likes the muscle oxygen monitor because it lets him confirm two of them directly: tension shows up as occlusion, and metabolic stress shows up as oxygen desaturation, so he can prescribe the exact rep count that reaches the trigger.

What is selective hypertrophy of fast twitch fibers?
It is growing the fast twitch fibers specifically rather than adding size across the board. Davis calls it a coaching Holy Grail and points to shot putter Werner Gunther, who on paper looked less fast twitch than his brother yet built enormous fast twitch fibers through years of ballistic, plyometric, and heavy training.

About the authors

Aaron Davis is a sports performance and health coach with Train Adapt Evolve in Austin, Texas. A former 800 meter runner at Adams State and a track coach who later worked in the CrossFit and private performance world, he uses physiological tools, including muscle oxygen monitoring, to individualize how his athletes train and recover.

Joel Smith is the host of the Just Fly Performance Podcast and the founder of Just Fly Sports, a former collegiate strength and track and field coach who focuses on speed, power, and athletic development. Listen to the full episode with Aaron Davis on Just Fly Sports.